1. Field of the Invention
The present invention relates to a vapor source holding container for the evaporation by heating of an evaporative material such as, for example, a material composed of selenium and tellurium. The container is constructed so that the vapor can be led out through an opening smaller in size than the evaporating area.
2. Description of the Related Art
Vapor source holding containers are called evaporating boats. Those that have a top opening of almost the same size as the evaporating area of a vapor source and are called open boats. However, it is difficult to control the evaporating rate in an open boat. In use this can cause abrupt boiling of the vapor source. This problem exists even if an open boat were composed of a low emissivity material.
On the other hand, the so-called Knudsen cell type container is free from this drawback. Such containers are excellent because the top opening is smaller than the evaporating area. Therefore, the evaporating rate is effectively controllable. If vapor bursts by an abrupt boiling, before arriving at the top opening, it attaches to the wall portion, so that the vapor never flies out toward the vacuum deposition-receiving substrate.
However, it has now been found that when the heat emissivity of the external surface of the boat, particularly of the external surface of the wall portion that is in contact with the vapor source is great, the variation in the temperature distribution increases in the longitudinal direction of the vapor source due to the heat radiation from aforesaid surface and also in the vertical direction of the vapor source. Such variations of the temperature distribution produce unevenness in the evaporation, thus leading to unevenness in the thickness and characteristics of the resulting deposition layer. This results in lowering the yield of the product.
When the heat emissivity of the external surface of the boat is low, the amount of heat released from the wall portions is low. Therefore, immediately after the heating begins, the temperature rises so quickly that it easily exceeds the settled evaporating temperature. If the heater is then turned off, the temperature then becomes lower. If this is followed by continued temperature controls, the temperature hovers around the evaporating temperature. This is called the hunting phenomenon. The large variation of temperature distribution causes unevenness in the thickness and characteristics of the deposition layer. Therefore, the yield rate of the products is lowered. Further, the temperature response temperature control becomes worse because it is hard for the heat to be emitted from the boats. Therefore, it requires many hours to reach and stably obtain the given evaporating temperature. Furthermore, the boat's cooling rate after completion of the evaporation is as low. This significantly lowers the efficiency of the evaporating operation in addition to providing the difficulty in the temperature control mentioned above.
Phenomenalistic investigations of a vapor source in a fused state have shown that a density convection is caused in the fused solution or a convection generated by the difference of the surface tension in the surface region of the fused solution causes the composition of vapor to become unstable. This is especially true when the vapor source is composed of an alloy, so that the density distribution of a deposited layer is dispersed.
Therefore, it is desirable to be in a state where the described phenomena can be prevented, and namely where a deposition is to be made under such a condition that the Rayleigh number and the Marangoni number of an alloy-fused solution are not greater than 1700 and 100, and more preferably not greater than 100 and 50, respectively, provided that the above numbers are defined as the following formulas: ##EQU1## wherein, g: acceleration of gravity
.beta.: thermal expansion coefficient PA1 l: depth of solution PA1 .nu.: kinematic viscosity PA1 .kappa.: thermal diffusibility PA1 .gamma.: surface tension PA1 T: temperature PA1 .mu.: viscosity PA1 .DELTA.Tl: temperature gradient per unit length in the direction of depth of solution PA1 .DELTA.Ts: temperature gradient between arbitrary two points of the surface of solution